The introduction of optical information into plastic materials through laser radiation is known per se. For this purpose, one differentiates between laser marking and laser subsurface engraving.
The identification of plastic through laser marking acts on the object surface and/or in the area proximal to the surface. For this purpose, the absorption of the laser energy in the plastic material through direct interaction with the polymer or with an additive added to the plastic material, such as an organic coloring agent or an inorganic pigment, which absorbs the laser radiation, is decisive. In any case, a chemical material change and therefore a locally visible discoloration of the plastic is caused through absorption of the laser energy.
Laser-markability is a function of the wavelength-specific absorption behavior of the plastic materials and/or the polymers on which they are based, of the wavelength-specific absorption behavior of any laser-sensitive additives, and of the wavelength and radiated power of the laser radiation to be used. In addition to CO2 and excimer lasers, Nd:YAG lasers (neodymium-doped yttrium-aluminum-garnet lasers), having the characteristic wavelengths of 1064 nm and 532 nm, are increasingly being used in this technology. Laser-markable plastic materials, which contain laser-sensitive additives in the form of coloring agents and/or pigments, generally have a more or less pronounced coloration and/or translucency. The molding compounds which are to be implemented as laser-absorbent are often thus equipped by introducing carbon black.
High-transparency plastic materials, which may be made laser-markable by adding nanoscale laser-absorbent metal oxides, are described in German Utility Model 20 2004 003362.3 and in German Patent Application 10 2004 010504.9, which has not previously been published. The publications DE 44 07 547 and U.S. Pat. No. 5,206,496 are cited as examples of the technology of subsurface engraving of glasses and plastics transparent to laser radiation by laser beams. (All of these references are hereby incorporated by reference in their entirety.) In contrast to laser marking, laser subsurface engraving acts at any arbitrary depth of the material. This requires that the material be essentially transparent to the incident laser radiation, since this would otherwise be absorbed in the surface region.
During the focusing of a laser beam of sufficiently high power density in the interior of the material, which is transparent to laser light per se, there is a limited development of thermal energy in the laser focus because of optical effects. This heat development results in local, narrowly limited microcrack formation in the material. Microcracks of this type have a point diameter of 25-40 μm. In glasses and plastics which are transparent in visible light, the microcracks appear as bright points because of the scattering of the daylight at the crack edges.
Through the deflection of the laser radiation via mirrors and the movement of the workpiece and through synchronization between the movement sequence and the laser pulses, corresponding structures made of individual microcracks may be assembled in the workpiece. The pulse sequence frequency of the laser typically used for this purpose allows the production of structures having up to approximately 1000 points per minute. The starting point is a 3-D illustration of the later motif in a CAD program. The surface or the entire structure of the model is resolved as a point cloud by a computer, whose individual points are implemented as microcracks in the glass or plastic by the laser beam. The denser the point cloud through which the object is illustrated, the more precise and cleaner will the model be imaged.
During the laser subsurface engraving of plastics using laser light to which the plastic is transparent, marking in the interior of the material in the form of microcracks is produced through corresponding focusing of the laser beam. Uncontrolled crack formation and crack propagation may occur in this case. This represents a weakening of the material. Keeping this weakening as small as possible is desirable.
In glass, crack formation may result in later destruction of the molded body, which sometimes only occurs days or even weeks after the laser subsurface engraving. In plastics, in addition to crack formation, local destruction of the material and carbonization may occur, which is undesirable in the subsurface engraving of material transparent in visible light because of the dark discoloration. A further problem of laser subsurface engraving using methods and materials according to the related art is the inadequate imaging precision of detailed, filigree patterns, both in glass and in plastics. Theoretically, the imaging precision may be improved by increasing the point cloud density. However, at a certain density, the points run into one another due to uncontrolled crack propagation and are no longer resolved, so that the imaging precision even suffers.
A method for crack-free laser subsurface engraving through laser pulses in the femtosecond range is described in U.S. Pat. No. 5,761,111. However, lasers suitable for this purpose are not yet widely available for technical use and would be very expensive.
In U.S. Pat. No. 6,537,479, the problem of crack formation is avoided in that the laser marking is performed in the plasticized state of the material and the object is either left in this state (enclosed by a solid protective envelope) or subsequently hardened. This method is very complex and additionally, in the case of subsequent hardening, is restricted to materials which do not display shrinkage upon hardening, since otherwise the filigree geometry of the laser engraving would be destroyed. This method has limited use and is very time-consuming.